In the present era of diminishing petroleum reserves and political instability in petroleum-rich countries, industrial society must develop ways to utilize the world's abundant and renewable biomass resources to provide new sources of energy and chemical intermediates (1). For example, the transportation sector requires fuels that can be efficiently converted to energy and that possess high energy densities. The chemical industry requires functional molecules such as olefins (e.g., ethylene, propylene) and aldehydes (e.g., formaldehyde) that can be used to make polymeric materials. A feature that makes biomass-derived carbohydrates a particularly promising class of compounds to supplement (or in some cases replace) petroleum in the aforementioned areas is that the stoichiometric building block in these compounds has an atomic composition of H:C:O equal to 2:1:1. Thus, carbohydrates are ideal candidates to be converted to H2/CO gas mixtures. These types of gases are commonly called “synthesis gas,” or simply “syngas.” Synthesis gas can be converted by Fischer-Tropsch synthesis over Fe-and Co-based catalysts (2) to yield long-chain linear alkanes for use as diesel fuel. Synthesis gas can also be converted over Cu-based catalysts (3) to yield methanol for use as a feed to produce olefins, formaldehyde, and gasoline.
While producing synthesis gas from biomass has been recognized for years as a promising platform from which a variety of valuable products can be made, conventional routes to produce synthesis gas from biomass are not terribly efficient because of the high temperatures required. For example, direct catalytic gasification of biomass requires a temperature of 800 K and higher (4). Two-stage gasification of biomass likewise requires high temperatures: a fast pyrolysis of biomass (at about 773 K), followed by a steam reforming of the resulting bio-oil (at about 1000 K) (5, 6). Moreover, gasification of biomass typically leads to a complex set of byproducts, including tar (volatile organics), char (solid carbonaceous materials), and light hydrocarbons, as well as NOx and SOx compounds produced during high temperature combustion processes (1, 4-6).
A relatively recent, and rapidly growing, use of biomass is in the production of bio-diesel fuel via the trans-esterification of vegetable oils and animal fats (1, 7, 8). The trans-esterification reaction yields a low-value waste stream of glycerol that often contains glycerol-in-water concentrations from 50 to 80% (8). The resulting glut has caused the U.S. price of glycerol to tumble from roughly $2,100 per metric ton in 1995, to less than $1,000 per metric ton in 2003 (for USP-grade 97% glycerol, prices supplied by Procter & Gamble). The current (2006) production of bio-diesel in the U.S. and Europe is 1×108 and 2×109 liters per year, respectively. Due to tax credits and other economic incentives provided by several national governments, these quantities are expected to double in the very near future (8, 9). Regarding bio-diesel fuel tax credits in the U.S. see IRS Publication No. 378, and sections 6426(c), 6427(e), and 40A of the Internal Revenue Code.
Glycerol can also be produced by fermenting sugars such as glucose (10). Unlike fermenting glucose to yield ethanol, which produces ethanol at concentrations of only about 5 wt % in water, fermenting glucose to yield glycerol can produce glycerol at concentrations near 25 wt % (10). This higher concentration of glycerol compared to ethanol decreases the energy costs required to remove water from the oxygenated hydrocarbon fuel. Indeed, one of the most energy-intensive steps involved in the production of fuel-grade ethanol from glucose is the distillation step (11, 12). Another route for production of glycerol from glucose and other sugars is through hydrogenation of glucose to sorbitol (13, 14), followed by hydrogenolysis of sorbitol to polyols having lower molecular weights (15, 16).
In previous work co-authored by some of the present inventors (17-19), it was shown that solutions of polyols in water (e.g., ethylene glycol, glycerol, sorbitol) could be converted by aqueous-phase reforming to H2/CO2 gas mixtures containing low levels of CO (e.g., 500 ppm) over supported Pt catalysts at temperatures near 500 K. This aqueous-phase reforming process gives rise to low CO:CO2 ratios in the effluent gas stream because the water-gas shift (WGS) reaction is highly favored at the high partial pressures of water generated under these reaction conditions (e.g., 25 bar). Thus, aqueous-phase reforming reaction conditions are not favorable for producing synthesis gas, where high CO:CO2 ratios are required. Other researchers have studied the vapor-phase reforming of glycerol. Czernik et al. (20) reported high selectivity for producing H2 by steam reforming of glycerol at high temperatures (1023 K) over a commercial Ni-based naphtha reforming catalyst. Suzuki et al. (21) also observed high selectivity for production of H2 by steam reforming of glycerol at high temperatures (873 K) over a 3% Ru/Y2O3 catalyst, but they employed a high space velocity of sweep gas in the experiments. Therefore, a better catalytic system for vapor phase conversion of glycerol at low temperatures remains to be developed.
The Fischer-Tropsch (F-T) synthesis for producing synthetic hydrocarbons from synthesis gas is well known. It was first implemented on a large scale by the Germans during World War II to make liquid fuels from coal. The general F-T reaction is as follows:CO+2H2→—(CH2)—+H2O H=−167 kJ/mole  (1)where —(CH2)— represents the basic building block of the hydrocarbon products. The FT synthesis is highly exothermic, which leads to heat transfer being a significant factor in the design of an F-T reactor.
A good deal of research has been done on maximizing the synthetic possibilities of the F-T reaction. See, for example, U.S. Pat. No. 6,696,501, which describes a method for converting natural gas or other fossil fuels into higher hydrocarbons. Here, the method uses a combination of steam reforming of fossils fuels to yield synthesis gas, followed by a F-T synthesis and a second steam reforming of the tail gas. The reformed tail gas is then fed back into the F-T reactor.
See also U.S. Pat. No. 6,976,362, which describes a method of integrating synthesis gas generation, an F-T reaction, and a water-gas shift reaction, to yield CO2, aliphatic hydrocarbons, and hydrogen, and then burning the hydrogen in a gas combustor turbine to generate electricity.
As briefly noted above, an important parameter for determining the theoretical maximum yield of synthetic hydrocarbons in a F-T reaction is the stoichiometric number SN, defined as:SN=(H2—CO2)/(CO+CO2)  (2)Theoretically, the yield of synthetic hydrocarbons is at its highest when SN=2.0 and CO does not react further to form CO2 via the water gas shift reaction. In this case, the H2/CO ratio will be equal to SN, i.e. 2.0, which theoretically gives the highest yield of synthetic hydrocarbons.
Biomass is comprised primarily of carbohydrates (e.g., starch and cellulose). One method to convert these compounds to liquid fuels is by fermentation to produce liquid alcohols, such as ethanol and butanol. The technology to convert grain-derived starches to ethanol via hydrolysis, fermentation, and distillation is well established, and advances are being made in the cost-effective conversion of lignocellulosics to ethanol (e.g., through the development of new enzymes for cellulose hydrolysis). The advantages of ethanol as a transportation fuel are that it is a liquid and it has a high octane number (a research octane number of 130). However, ethanol has several notable inherent disadvantages as a fuel as compared to long-chain alkanes: (i) ethanol has a lower energy density compared to petroleum (i.e., approximately 20×103 BTU/liter for ethanol versus 30×103 BTU/liter for petroleum); (ii) ethanol is completely miscible with water, leading to significant absorption of water into the fuel; and (iii) it has a relatively low boiling point (73° C.), leading to excessive evaporation at elevated temperatures. Most significantly, however, the fermentation process used to produce bio-ethanol from carbohydrates leads to an aqueous solution containing only about 5 to 10 wt % ethanol. A significant amount of energy is required to distill the ethanol from the water to yield fuel-grade ethanol. Indeed, the overall energy balance for production of bio-ethanol is not very favorable, and it has been estimated that the amount of energy required to produce bio-ethanol is approximately equal to (or greater than) the energy-content of the ethanol produced (11, 12, 36).
Long-chain alkanes comprise the vast majority of components in transportation fuels from petroleum (branched alkanes in gasoline, linear alkanes in diesel). Converting renewable biomass resources to liquid alkanes is therefore an attractive processing option. Most notably, liquid alkanes produced from biomass (i) can be distributed using infrastructure already employed for petroleum-derived products; (ii) can be added to the existing petroleum pool for further processing (e.g., blended fuels); and (iii) can be burned in existing internal combustion engines, without altering the engine or the fuel.